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United States Patent |
5,068,582
|
Scott
|
November 26, 1991
|
Brushless pulsed D.C. motor
Abstract
A commutated brushless dc motor includes a detector which senses the zero
crossings of the unenergized winding. A timing system includes a reference
capacitor coupled to the detection unit and charged to a voltage
proportional to zero crossing time. The voltage is compared to a capacitor
charged from a constant current source. At a percentage level of the
reference capacitor, a commutating pulse is applied to a winding
commutating circuit. The incoming pulses drive a counter and actuates a
programmed decoder which generates output signals for commutation of the
windings and selection of the next unenergized winding. The detector is
selectively connected to sense the induced voltage in the unenergized
winding. To start the motor, a separate charging circuit creates initial
pulses to energize the windings and accelerate the motor. At a given
speed, the zero crossing circuit is enabled. An integrated circuit system
has the zero crossing signals applied to a resettable counter which counts
at a fixed rate between zero crossings. The count is latched and is
proportional to the time between zero crossings. The counter signal and
the latched signal are compared to generate the commutating pulse.
Inventors:
|
Scott; Kenneth C. (San Diego, CA)
|
Assignee:
|
A. O. Smith Corporation (Milwaukee, WI)
|
Appl. No.:
|
529920 |
Filed:
|
May 29, 1990 |
Current U.S. Class: |
318/254; 318/138 |
Intern'l Class: |
H02P 006/02 |
Field of Search: |
318/138,254,439
|
References Cited
U.S. Patent Documents
4027215 | May., 1977 | Knight | 318/341.
|
4238717 | Dec., 1980 | Knight | 318/341.
|
4467320 | Aug., 1984 | McPhee | 340/347.
|
4492902 | Jan., 1985 | Ficken et al. | 318/254.
|
4492903 | Jan., 1985 | Knight | 318/341.
|
4495450 | Jan., 1985 | Tokizaki et al. | 318/138.
|
4528485 | Jul., 1985 | Boyd, Jr. | 318/138.
|
4642537 | Feb., 1987 | Young | 318/254.
|
4835448 | May., 1989 | Dishner et al. | 318/254.
|
4922169 | May., 1990 | Freeman | 318/254.
|
Primary Examiner: Ro; Bentsu
Attorney, Agent or Firm: Andrus, Sceales, Starke & Sawall
Claims
I claim:
1. A commutated brushless dc motor comprising a stator with a plurality of
circumferentially spaced stator windings, a rotor coupled to said stator
and rotatably mounted to rotate relative to said stator winding as the
result of energization of said windings, a dc power supply to establish
pulsed energization of said windings in sequence with at least one winding
de-energized, an alternating current induced signal being induced in said
de-energized winding and having periodic zero crossings, a timing device
coupled to the stator to monitor the time between the zero crossings of
said induced signal and generating an electrical zero crossing signal
proportional to such time, a signal latch unit coupled to said timing
device and storing the zero crossing signal as a latched signal, a reset
unit connected to said timing device to initiate a new timing cycle to
generate a next zero crossing signal, and a comparator connected to said
timing device and said latch unit and responsive to a proportion of the
latched signal to generate said pulsed energization of said windings in a
continuous repetitive sequence.
2. In the brushless motor of claim 1 wherein said induced signal has an
alternating period of 360.degree. and said stator includes three windings,
each winding being commutated "on " to initiate pulsed energization at
30.degree. in the induced signal and the respective related rotor rotation
and commuted "off" at 150.degree. to terminate said pulsed energization
such that the three windings are sequentially pulsed energized for
successive periods of 120.degree. in each 360.degree. rotation of said
rotor.
3. In the motor of claim 1 wherein said timing device is a timing capacitor
and a constant current charging circuit actuated by successive zero
crossing signals and thereby establishing a timing capacitor charge equal
to the period between zero crossings.
4. The motor of claim 3 wherein said timing device includes a reference
capacitor coupled to said timing capacitor and set to a reference voltage
equal to the voltage of the timing capacitor at each zero crossing, said
comparator being connected to said timing capacitor and to said reference
capacitor and creating a switching signal in response to a charge in said
timing capacitor equal to a percentage level of the reference voltage on
the reference capacitor.
5. The motor of claim 4 including a zero crossing detector having detection
switch means to sequentially monitor the zero crossing of the stator
windings, power switch means connected to sequentially connect said stator
windings to power, a programmed decoder connected to said comparator and
decoding the switching signals and generating power output signals to said
power switch means for commutation of the stator windings and control
signals to said detection switch means of the zero crossing detector to
select the unenergized winding for monitoring the next zero crossing.
6. The motor of claim 5 wherein said comparator includes a first input
connected to said timing capacitor and a second input, a voltage dividing
network connected to said reference capacitor and having a reference
output line connected to said second input, said comparator responding to
a voltage on the timing capacitor equal to the reference voltage on said
reference output line to generate said switching signal, a monostable
circuit connected to said comparator to generate a sharp pulse signal.
7. A brushless dc motor having a rotor and a stator with at least three
equicircumferentially spaced windings connected in a wye configuration in
with a common center line and connected to a switched dc power supply and
having separate switch units for selectively energizing said windings and
thereby causing rotation of said rotor, said rotation of said rotor
inducing an induced electrical signal in said windings, comprising a
sensor coupled to said windings and responsive to a zero crossing of the
induced electrical signal in said windings, a signal generator creating a
control signal having a characteristic varying from a reference level
proportionately with time, said signal generator including a reset unit
for resetting said generator to said reference level, a control reference
unit connected to said signal generator and set to correspond to said
control signal and having an output corresponding to a selected percentage
of the control signal, a trigger signal source connected to the output of
said control reference unit and to said signal generator operable to
establish a switching signals with said signal generator at a signal
output equal to said selected percentage of said control signal, and said
sensor being connected to said signal generator and operable to reset said
signal generator at each of said zero crossings and thereby repetitively
establish said control signal proportional to the period between
successive zero crossing, and a circuit for connecting of said trigger
signal source to said separate switch units and thereby establish
sequential actuation of said separate switch units and pulsed energization
of said windings.
8. The motor of claim 7 wherein said signal generator includes a power
supply, a timing capacitor connected to said power supply and charged at a
constant rate with time to establish said control signal, said sensor
includes a zero crossing detector connected to said windings for
generating a zero crossing signal at each zero crossing of said windings,
said reset unit including a reset switch connecting said timing capacitor
to said reference level at each zero crossing, a reference capacitor, a
switch connecting said reference capacitor to said timing capacitor and
setting said reference capacitor to a reference voltage, a proportional
signal decoder coupled to said reference capacitor and establishing a
control reference signal proportional to a fixed percentage of said
reference voltage, said trigger signal source coupled to said proportional
signal decoder and to said timing capacitor and generating a switching
signal when said control signal of said timing capacitor corresponds to
said reference signal.
9. The motor of claim 8 including a sequence controller for establishing
sequential and cyclical energizing of said windings and for sequential and
cyclical sensing the induced signal in said windings.
10. The motor of claim 7 including a counter having a reset element for
setting the counter to a reference count and counting at a fixed rate from
said reference count, said sensor connected to said windings and
establishing a reset signal at each zero crossing of said induced signals
in said windings, said sensor being connected to said reset element to
reset said counter to the reference count at each zero crossing, a latch
count unit connected to said counter and set to one half the count in the
counter at each reset of the counter, a comparator to compare the count in
said counter and in said latch count unit and operable to generate a
trigger signal when the count in said counter equals the count in said
latch count unit whereby said windings are sequentially energized for
fixed periods related to the periods between said zero crossings and
establishing a smooth continuous rotation of said rotor.
11. A controlled power supply for energizing a brushless dc motor having a
rotor with circumferentially spaced magnetic poles and a stator with
circumferentially spaced windings, comprising a switching device connected
to said windings and having a plurality of input controls for sequentially
energizing said windings and establishing an interacting magnetic field
sequentially stepped about said rotor and thereby establishing rotation of
said rotor, at least one of said windings being de-energized and having an
induced signal induced therein as a result of the rotation of said rotor,
said induced signal varying in a periodic manner with spaced zero
crossings in accordance with the rotation of said rotor, a detector
coupled to said windings and sequentially operably coupled to the
de-energized windings to sense the induced signal in successive windings
and thereby the position of the rotor relative to the stator, a timing
device connected to said detector and generating a control signal directly
proportionally to the period between said zero crossings, a signal
dividing unit connected to said timing device and generating a fixed
percentage signal for each successive control signal, a comparator
connected to the signal dividing unit and to the timing device and
generating a trigger signal with said control signal being equal to said
fixed percentage signal, and a controller connecting said comparator to
said input controls to actuate the switching device to sequentially
energize the windings in response to the sequence of said trigger signals.
12. A brushless dc motor having a permanent magnet rotor and a stator with
a plurality of equicircumferentially spaced windings having one inner end
of the windings connected to a common line and each of the outer ends of
the winding connected to a dc power supply, said windings being
sequentially energized with dc power positively supplied to one of said
windings and returned to said supply through a second of said windings,
and said positive power being sequentially applied to said windings in one
direction for forward rotation of said motor and in a reverse direction
for reverse rotation of said motor, said rotor inducing an alternating
sinusoidal induced voltage in the unenergized winding to be next
energized, said induced voltage having two zero crossings in each voltage
cycle, the improvement in a control for switching of said power supply to
said windings, comprising a zero crossing detecting network connected to
said windings for detecting the zero crossing of each induced voltage in
said unenergized winding, a voltage reference line, said detecting network
including a resistive coupling network including a separate first voltage
resistor dividing branch connected to the end of each winding and to said
voltage reference line and including a second voltage resistor dividing
branch connected to said common line connection and said voltage reference
line, a gate unit connected to each of said resistor dividing branches,
said gate unit having a common output line, a first comparator amplifier
having first input connected to said common output line and having a
second input connected via the second voltage resistor dividing branch to
said common line connection, an inverted amplifier to invert the output
signal of said first comparator amplifier, a second comparator amplifier
having a first input connected to said voltage reference line and having a
second input, first and second transfer switches connecting said first
comparator amplifier and said inverter amplifier to said second input of
said second comparator amplifier, three detection switches connected one
each to each of the first voltage resistor dividing branches, first and
second transfer switches, a programmer coupled to sequentially actuate
said three detection switches in sequence to sequentially couple the
output of the windings to said first comparator amplifier and to
alternately actuate said first and second transfer switches for
alternately actuating such first and second transfer switches to establish
a positive input signal to said second comparator amplifier and thereby
generate a pulse output signal at each zero crossing of the unenergized
winding and establishing a series of pulse signals precisely identifying
the zero crossing points in the sinusoidal induced voltage in the
unenergized winding and thereby the position of said rotor relative to
said stator windings, a measurement and timing circuit including a timing
device to create a time signal proportional to the period between
successive zero crossing in said unenergized windings, a storage device to
store the time signal and including a proportional output element to
establish a reference signal equal to a percentage of said time signal
which equals to the period between the last zero crossing and the next
winding energization, a trigger comparator connected to said timing device
and to said storage device and establishing a switching signal to said
programmer in response to the input of said time signal and said reference
signal.
13. The motor of claim 12 wherein said measurement and timing circuit
includes a reference capacitor and a timing capacitor, a constant current
source, said timing capacitor being connected to said constant current
source for charging said timing capacitor at a preselected constant rate,
a first gated switch connected to said timing capacitor to discharge said
timing capacitor, a second gated switch connected to said reference
capacitor and to said timing capacitor to set said reference capacitor at
the charge level of said timing capacitor, said first and second gated
switches having inputs connected to the output of said zero crossing
detecting network and having said second gate switch actuated for a
momentary period prior to actuation of the first gated switch to thereby
set the reference capacitor to the charge set on the timing capacitor and
to immediately thereafter discharge the timing capacitor, a reference
voltage dividing network coupled to said reference capacitor and
establishing a proportional reference voltage signal equal to the voltage
created in said timing capacitor in a selected rotation of said rotor,
said trigger comparator having a first input connected to said reference
voltage dividing network and a second input connected to said timing
capacitor, said trigger comparator establishing an output when the output
of said timing capacitor equals the proportional signal from said
reference voltage dividing network.
14. The motor of claim 13 wherein said measurement and timing circuit
includes a starting circuit for generating a fixed series of pulses
independent of said pulse output signal at each zero crossing to initiate
operation of said motor to establish operative zero crossing signal
pulses, comprising a start voltage dividing network connected to said
power supply, a start comparator having a first input connected to said
start voltage dividing network and to said timing capacitor, the output of
the start comparator developing a train of pulses for controlling the
starting of said motor, an OR gate connected to said start comparator and
to said trigger comparator and thereby establishing a train of pulses when
generated from either of said start comparator or said trigger comparator,
said starting circuit being established whereby said timing capacitor is
reset by said zero crossing signal pulse prior to the level on said timing
capacitor rising to the level for actuating said start comparator.
15. The motor of claim 13 wherein said programmer includes a recycling
counter to establish a series of pulse signals with one pulse for each
zero crossing of said induced voltage during each revolution of said motor
and to repetitively count said pulses in sequential count cycles, said
programmer including a decoder to decode the pulse position in each said
count cycle and establish sequential actuation of the switches of said
zero crossing detecting network to sequentially activate the unenergized
winding, said decoder including switch output lines coupled to actuate
said detection switches and said first and second transfer switches to
selectively supply the power to said windings in sequence.
16. The motor of claim 15 wherein said power supply includes positive power
switches, one for each of said windings and connected between said power
source and the end of said corresponding winding, return switches
connected between said positive power switches and said power supply to
form a return path for the power through one and a second winding, said
decoder actuating one of said positive power switches and one of said
return switches to establish the flow through said windings.
Description
BACKGROUND OF THE PRESENT INVENTION
This invention relates to a pulsed brushless direct current (dc) motor and
particularly to such a motor including an electronic control for
energizing of the motor to maintain motor operation in the presence of
varying motor torques, available line voltages and the like.
Brushless dc motors have been developed for precise precision drives in
various applications. Brushless dc motors, for example, are now widely
used in disk drives for computing systems. In such motors, a rotor
includes fixed polarity poles. The stator winding includes a plurality of
spaced windings. Energizing of the windings in sequence with a dc signal
generates an interacting field with that of the rotor poles to establish
rotation of the rotor. The pulsing of the stator windings at predetermined
orientations of the rotor with respect to the stator windings is necessary
to provide maximum torque per power pulse application of the windings, and
is critical in order to maintain and establish a desired speed
characteristic with varying torque, voltages and the like. A widely used
system includes Hall cell sensors mounted to sense the position of the
rotor relative to the stator. The output of the Hall cell produces an
appropriate signal to the control system for switching power from one
winding to another at the proper commutation time for the windings. The
Hall cell elements of course contribute to the cost of the motor, which is
further increased because of the necessity to provide precise and accurate
mounting of the sensors as well as the additional leads from the motor
proper to the electronic control system. Generally, the system requires a
specially shielded lead, and each motor is more or less constructed as a
custom design and does not lend itself to building of a single standard
motor line. Hall cell elements are semi-conductor based structures,
therefore have temperature limitations which must be considered, and
inherently raise a problem with respect to reliability of the motor
operation. Motor derating may be specified to anticipate adverse
temperature conditions.
An alternative sensing system which has been suggested is based on an
electronic sensing system including directly sensing the electrical
characteristic in an unenergized winding of the stator for detecting the
desired time for commutating the motor windings. U.S. Pat. No. 4,027,215
which issued May 31, 1977 discloses a zero crossing detection system for
brushless dc motors to generate commutating signals. That patent also
discusses the background of dc and A.C. machines and further discusses
some of the disadvantages of the brushless dc motors which rely on
separate sensors for detecting rotor position and thereby the appropriate
commutation times. The '215 patent provides a two stage counter controlled
by the output of a signal induced in the unenergized winding for purposes
of detecting a proper commutation time. Thus, in a three phase stator
winding, the three power coils are equicircumferentially spaced or wound
on the stator. Each winding is pulsed during the period when the other two
windings would generate a signal less than the signal of the selected
winding. Thus, assuming a three phase time sequence with a sine wave
characteristic, each winding would be energized during its positive half
cycle and particularly between the period between 30.degree. and
150.degree. of such half cycle, or for the 120.degree. of each cycle of
rotation, with successive windings being energized sequentially to
energize only two windings simultaneously. The system used therein detects
the zero crossing and then activates a pair of counters for each phase to
monitor the zero crossings and the particular time of commutation. Actual
commutation is created by a plurality of solid state switches
interconnecting of the motor winding dc supply. The electronic control
system as disclosed in that patent provides a high degree of complexity to
avoid the necessity of the separate position sensing elements. The use of
the up-down counters provides for counting of the unit at one rate, such
as counting up at one rate during the zero crossing and then counting
downwardly at such rate to generate a pulse at a particular desired delay
time period and then recycling of the system to maintain the continued
operation of the two stage counter system. Until the motor reaches a
certain selected speed of operation, the unenergized winding does not
provide an appropriate signal for monitoring and controlling commutation.
The motor is therefore started with a time spaced pulsed control with
sufficient energy supply to initiate the operation of the motor as a more
or less conventional stepping motor wherein the windings are sequentially
energized to start the motor and accelerate the motor to a speed wherein
an effective detectable voltage appears in the unenergized winding to
permit the continued for operation with the zero crossing detection. Other
controls are also disclosed in the patent to provide for varying of the
speed and centering of the pulses to maintain maximum efficiency and the
like.
SUMMARY OF THE PRESENT INVENTION
The present invention is particularly directed to a commutated brushless dc
motor providing a simple, effective and reliable sensing of the zero
crossings and generating commutating pulses based on the zero crossing
detection.
Generally in accordance with the present invention, a timing device is
coupled to the motor stator and is reset at each zero crossing. The timing
device begins to monitor the time until the next zero crossing and
generates an electrical signal proportional to such time. The zero
crossing signal is latched or recorded at the next zero crossing and the
latched signal used for establishing the next commutating pulse, and
essentially simultaneously resets the timing device to initiate a new
recycle to record the next period of the zero crossing. The system repeats
the above sequence with continuous repetitive recycling, latching and
recycling of the timing device. The commutating control responds to a
proportion of the latched signal to generate the desired commutating of
the stator winding. Thus, with the three phase dc brushless motor, each
winding is commutated "on" at the 30.degree. angle in the respective
related rotor rotation and commutated "off" at 150.degree. such that the
three windings are sequentially commutated "on" for successive periods of
120.degree..
The system is particularly adapted to a very simple, reliable and effective
electronic control and can be incorporated into a capacitor based
electronic control, an integrated circuit electronic control or the like.
In one embodiment of the present invention, a capactive charging circuit is
provided for monitoring of the zero crossing time and the generation of a
commutating signal at an appropriate time based on the total capacitor
charge during the zero crossing time. In particular, the referenced
capacitor is coupled to a zero crossing detection unit input and is
triggered at each zero crossing to record the voltage created at the last
zero crossing. More particularly, a timing capacitor is coupled to a
constant current source. The capacitor is reset at each zero crossing,
with essentially immediate initiation of the next capacitor charging
cycle. The timing capacitor is charged at a constant rate related to the
constant current source. When the charge equals a percentage level of the
stored charge on the reference capacitor, a detection circuit conducts and
generates a commutating pulse. The commutating pulse is applied to an
appropriate commutating circuit to control the commutation of the several
windings. The unit can use a simple counter to detect the incoming pulse
signals which identify the on/off times for each of the individual
windings in sequence. The output is applied to a programmed decoder which
generates output signals for commutation of the windings and also actuates
the zero crossing detection circuit to select the unenergized winding in
appropriate sequence.
The zero detector includes a simple voltage sensing unit coupled to the
several windings and the reference or common connection to provide a
voltage related thereto with respect to such reference. The switch
connects each of the sensing units to a pulse generating circuit with the
switches activated in accordance with the winding energization to produce
an output for sensing the induced voltage in the unenergized winding. The
outputs are connected in common with a switching circuit for comparison
with the reference signal. Thus, whenever the switch is turned "on", it
provides an appropriate induced voltage signal to the comparator. When the
comparison of the sensed induced voltage and the references signal match,
a monostable circuit or the like is triggered to generate a sharp pulse
signal at the zero crossing. With the three winding motor, there will be
six pulses per rotor revolution. The pulses are connected as the input to
the commutating angle generator which in turn generates the commutating
pulses to the commutation circuit as such. A counter provides a binary
output identifying the six pulses received and fed into the decoder for
generating of the appropriate pulses for sequential energizing the
windings and simultaneously resetting the zero crossing circuit to detect
the appropriate zero crossing of the unenergized winding.
Prior to the motor reaching at an appropriate speed to generate the
triggering pulses, a separate charging circuit is provided for
establishing an initial series of stepping pulses at a constant rate. The
initial charging circuit serves to sequentially energize the three
windings and accelerate the motor to approach operating speed, at which
time the zero crossing circuit is enabled to activate the motor under the
desired commutated mode. The stepping mode is established and maintained
at least until the motor reaches a sufficient speed to generate effective
zero crossing pulse signals from the zero crossing detector. This circuit
may include a comparator having its reference input set at a fixed
percentage voltage of the voltage generated by the triggering or timing
capacitor under normal commutated operation of the motor. The comparator
is automatically disabled once the zero crossing commutating mode is
instituted.
To incorporate the system into an integrated circuit, the zero crossing
signals are applied to an electronic counter. The counter is initialized
at a first zero crossing, which initiates a count at a fixed rate until
the next zero crossing. The counter therefore generates a time signal
directly related to the time between the successive zero crossings in an
induced winding. The counter data is latched into a suitable latch unit at
the end zero crossing and provides a count signal directly corresponding
to the total time between the zero crossings. This time essentially
corresponds to a time of 60.degree. in the revolution of the rotor. The
counter is simultaneously reset at the latch time and begins a new count.
When the running counter reaches a value equal to one-half of the latched
value, a commutating pulse is generated to commutate the windings, but the
counter continues to count to establish the next reference signal.
In the steady state, the system thus generates a stator flux vector which
steps about the machine in increments of 60.degree.. The vector leads the
actual rotor flux by an average angle of 90.degree. which corresponds to
the desired angle for maximum torque transfer. A slight angle error may be
introduced as a result of delays between measurement of the angle
positions as well as the resulting implementation based on such
measurements. The system however provides a closed loop control which
tends to automatically modify and correct for such error.
DESCRIPTION OF THE DRAWINGS FIGURES
The drawings furnished herewith illustrate the best mode presently
contemplated for the invention and are described hereinafter.
In the drawings:
FIG. 1 is a schematic circuit of the drive and control for a brushless
motor illustrating an embodiment of the invention;
FIG. 2 is graphically illustrating of the motor waveforms and the timing
control signals;
FIG. 3 is graphically illustrating of the switching and pulsing waveforms
resulting from the control signal of FIG. 2; and
FIG. 4 is a block diagram illustrating an integrated digital embodiment of
the invention shown in FIGS. 1-3.
DESCRIPTION OF THE ILLUSTRATED EMBODIMENT
Referring to the drawings and particularly to FIG. 1, a permanent magnet dc
motor 1 is illustrated including a rotor 2 coupled to a three-phase wye
connected stator unit 3. The stator unit 3 includes three separate
windings 4, 5 and 6 distributed in 120.degree. spaced relation. The motor
1, as schematically illustrated, is driven by sequential energization of
the three individual windings 4, 5 and 6. The magnetic field established
by each winding is coupled to the magnetic field of the rotor 2 to create
the turning force on the rotor for driving a suitable load, not shown. In
accordance with well known practice, the windings are driven from a
switched dc power supply 7.
In the illustrated embodiment, the three windings 4, 5 and 6 are shown
connected to the dc power supply 7 which includes a full wave rectifier 10
connected to an A.C. source 11. A switching bridge network 12 includes six
solid state switches 13, 14, 15, 16, 17 and 18, shown as switching
transistors. Pairs of the several switches 13-18, inclusive, are
simultaneously energized and turned on to sequentially connect the
windings 4, 5 and 6 to the dc power supply 7.
The particular sequence of activating switches 13-18 and energizing
windings 4, 5 and 6 is determined by the desired direction of rotation.
Thus, energizing of windings in the sequence of windings 4, 5 and 6
develops first or forward rotation and the reverse sequence of winding 6,
5 and 4 develops the opposite or reverse rotation as a result of the
creation of the corresponding magnetic poles in the stator relative to the
rotor.
A drive controller 19 is actuated by a trigger or winding switch signal to
turn-off an "on" switch and turn-on an "off" switch for the next winding
to be energized to establish the sequential energization of the windings,
as more fully developed hereinafter.
The controller may be a commercially available programmable controller
which sequentially transmits signals to the gates or turn-on input element
20 of the power switch transistor 13-18 in the proper sequence. The
sequence is set by a "forward/reverse" input unit 21 which may be manually
set as shown by a switch unit, or connected to a suitable condition
responsive control or the like. The controller includes individual
"gating" output lines 22 connected to the input terminals or elements 20
of the respective transistors 13-18, inclusive.
The sequential actuation pulsing of switches 13-18 establishes at least one
winding which, at any given instance, is connected to positive power and
one winding which is not connected to the power supply 7. The magnetic
field of the rotor 2 however cuts the unenergized winding and induces a
voltage and current flow in the unenergized winding which is detected for
determining the position of the rotor 2 relative to the windings. The
induced voltage is typically a sine wave, and with the illustrated three
phase winding configuration, a three phase induced voltage is created, as
typically shown in FIG. 2.
For optimum motor operation, each winding is energized for a period related
to the intersection of the several winding voltages in a three phase sine
wave system. Thus, each winding is energized and supplied with positive
power for 120 degrees between the 30 degree position and the 150 degree
point in each 360 degrees of rotation. The direct detection of the voltage
intersections is complex.
However, the intersections are directly related to the zero crossing points
of the several induced winding voltages as at 23 in FIG. 2, and by
precisely 30 degrees. In the present invention, a driver and timing
circuit 25 for the controller 19 generates a trigger signal directly
proportional to the time period between each zero crossing 23, and the
following 30 degree point of the monitored wave and is created by direct
monitoring of the zero crossing points 23. The zero crossing signal is
generated during each period by charging a storage capacitor 26 beginning
with each zero crossing and ending with the next zero crossing. The signal
is stored in a suitable signal storage unit, shown as a capacitor 24. A
trigger signal is generated equal to the fixed percentage of the stored
signal equal to the charge created during 30 degree rotation of the rotor
2. The new zero crossing related signal in capacitor 26 is compared with
the switch reference signal from capacitor 24, and when the new zero
crossing related signal equals the switch reference signal, a trigger or
switch signal is generated at an output line 26a and connected to the
controller to sequentially turn-off the power to one winding connected to
the power supply 7 and turn-on the power to the next proper winding of the
stator unit, with the cycle automatically being reinitiated and repeated.
Thus, the zero crossing related signal continues to build in the capacitor
26 to the next following zero crossing, at which time the signal is stored
in capacitor 24 to produce a new percentage signal level and capacitor 26
is reset and simultaneously again charged such that another new zero
crossing related signal is built in capacitor 26 to produce the next
succeeding trigger signal.
The output voltage of the reference capacitor 24 and the capacitor 26 are
connected to the opposite inputs of an amplifier comparator 27. When the
reference signal voltage is matched by the increasing voltage on the
capacitor 26, which is now being charged to record the next zero crossing,
the comparator 27 output switches to a high level as shown at 27a. The
high level signal is applied to a monostable circuit 28 30 to generate a
pulse signal 28a for signally switching of the windings. In the
illustrated embodiment of the invention, the pulse signal 28a is connected
to an OR gate 29 connected to output line 26a. The OR gate transmits the
signal to the controller 19 and thereby initiates actuation of the
switching circuit and changing energization from one winding to another.
The OR gate 29 is used to control sequential switching under start-up
conditions and normal operating condition. A turn-on circuit 29a is
provided for providing periodic pulse signals to the gate 29 for
initiating the start-up of the motor and until the rotor 2 speed is
sufficient to induce a signal in the unenergized winding for actuating the
drive circuit.
In the illustrated embodiment of the invention, the start-up circuit
includes a voltage dividing network 30 connected to the circuit supply,
with a fixed reference voltage line 31 from the network connected as a
first input to an amplifier comparator 32. The second input of the
comparator 32 is connected directly by a line 33 to the capacitor 26.
During the initial starting, the motor acceleration is controlled by the
charging of the compacitor 26 to the percentage of the constant reference
voltage from the voltage dividing network 30. The start comparator 32 thus
generates a series of step signals. A monostable unit 34 is connected to
the output of the comparator 32 and generates time spaced pulse signals
which are interconnected by a lead 35 to a second input of the OR gate 29.
The start-up circuit thereby provides a series of constant time spaced
signals applied to the controller 19 which operate the motor as a stepping
motor and accelerates the motor.
The trigger pulses 37 transmitted from the start-up circuit of the zero
crossing circuit via the gate 29 are supplied to the controller 19 which
actuates a counter 38 and creates a three bit signal for each of pulses in
groups of 6. Thus, each 360.degree. rotation of rotor 2 creates six zero
crossings and thus the required actuating of switches 13-18. A circuit
decoder is connected to the digital output of the counter and identified
each pulse within the sequence for each rotation. The circuit decoder 39
is set to initiate energization with one winding for forward rotation and
another for reverse rotation, as selected by operation of input unit 21.
The series of pulse signals 37 identifies which winding was last energized
and through the forward/reverse input 21 which winding is next to be
energized. The decoder 39 further provides information as to the next
unenergized winding 4, 5 or 6 to be monitored. Four signal selection lines
40 from the decoder 39 are coupled to a zero crossing detection circuit 41
and provide appropriate turn-on to the timing circuit 25 for selection of
winding 4, 5 or 6 for monitoring.
The three windings 4, 5 and 6 are shown connected in a wye or star
configuration, with a common or neutral reference connection of the three
windings tied to a dc common or reference line 43. The outer ends of the
three windings and the neutral line 43 are separately connected via a
similar coupling circuits 44 to the zero crossing detecting circuit 41.
Each coupling circuits 44 is a series resistors networks similarly
connecting each winding 4, 5 and 6 to ground or zero reference line 45 of
the circuit. Each network 44 includes a pair of voltage dividing resistors
46 and 47 to establish an attenuated induced motor voltage signal.
Separate signal lines 48, 49, 50 and 51 are similarly connected to each
signal tap at the common connection of the resistors 46 and 47 of the
circuits 44, respectively. A separate gated switch 52, 53 and 54 connects
each winding network or circuit 44 to a common signal line 55, which is
connected to one input of an amplifier 56. The common or neutral line
network 44 is connected to the second input of the amplifier 56 by a line
57. A gated switch 52, 53 or 54 for the unenergized winding 4, 5 or 6,
respectively, is turned on to monitor the induced voltage in the
corresponding winding for tracking the induced voltage passing through a
zero crossing. The switches 52, 53 and 54 are thus activated by the
controller 19 in the proper sequence and in timed relation to actuation of
the power switches 13-18. The zero crossing signal transmitted is the
portion of the induced voltage as shown at 57 in FIG. 2. FIG. 2
illustrates the sensed induced voltages in the unenergized windings 4, 5
and 6 at 58, 59 and 60, respectively. Thus, the windings sequentially are
actuated to generate the zero crossing signal at the output of the
amplifier 56. Every other zero crossing signal is negative going and the
tracked signal from the amplifier 56 is then negative going. Each negative
going signal is inverted by an amplifier 61 to provide positive going zero
crossing signals at each zero crossing.
Gated switches 62 and 63 selectively and alternately connect the outputs of
the amplifiers 56 and 61 to the input of an output amplifier comparator
64. The gated switches 62 and 63 are actuated in synchronism with the
switching of the windings to transmit the induced voltage during each zero
crossing from a first polarity, such as a positive to a negative polarity
and a negative to a positive polarity. The controller lines 40 include a
fourth line connected to switch 62. An inverter 65 connects the fourth
line 40 to the gate 63, and results in the alternate operation of gates 62
and 63.
Amplifier comparator 64 continuously compares the induced voltage with the
"0" reference and at the zero voltage of the sensed waveform an output
signal is generated. A pulse signal generator 65, shown as a monostable
circuit, is connected to the output of the comparator 64 and triggered by
the leading edge of the signal to generate a sharp zero crossing pulse
signal 66 in the form of a sharp pulse and, as separately shown applied to
the circuit 25, having a leading edge 66a and a trailing edge 66b. The
pulse signal 66 is coupled to the timing circuit 25 to selectively set the
charge and voltage in capacitor 24 and reset timing capacitor 26 to
initiate the start of a new zero crossing voltage and thereby establish
the series of triggering signals, as follows.
More particularly, the timing capacitor 26 is connected to a constant
current source 67 of any suitable construction and charged at a constant
rate. A reset gated switch 68 connects the capacitor 26 to a reference or
zero voltage line and is operable to reset the timing capacitor 26 to
reference, which is generally zero volts. The switch 68 is connected to
the zero crossing signal line to receive pulse 66, and reset the capacitor
26 to zero on the trailering edge 66b of pulse signal 66.
The reference capacitor 24 is connected into circuit and is set to the
voltage level of the timing capacitor 26 just prior to the reset of the
capacitor 26 to a reference zero level, as follows. A set switch 69
connects the reference capacitor 24 and to the zero crossing signal line
to receive the pulse signal 66. The switch 69 is a gated switch and is
turned on by the pulse signal 66. Actuation of the switch 66 completes a
path from capacitor 26 to capacitor 24, which sets the capacitor voltage
equal to the voltage of capacitor 26 just before capacitor 26 is reset.
The signal on capacitor 24 is thereby set at a level directly proportional
to the time between the last zero crossing and the current zero crossing,
and the voltage is established at the reference level for the next winding
to be fired.
A voltage divider network 70 including first and second resistors 71 and 72
is connected to the capacitor 24 to establish a percentage signal voltage
at a signal line 73 which is a fixed percentage of the voltage on
capacitor 24. Assuming a constant speed of rotor rotation, the fixed
percentage signal identified a particular amplitude of the rotor rotation;
namely, 30 degrees and therefore the position of the rotor PG,16 relative
to the windings. The signal at line 73 is therefore an accurate definition
of the time at which to switch the connection of one of winding 4, 5 and 6
to another winding to establish and maintain the sequential energization
of windings 4, 5 and 6 such as shown in FIG. 3, with each winding
energized for 120 degrees of rotation in each complete revolution of the
rotor 2, and more fully described hereinafter. The reference signal line
73 is connected to the negative input of comparator 27. The positive input
of comparator is connected to capacitor 26. When capacitor 26 reaches the
level of the voltage at line 73, the comparator 27 is driven on and
produces the signal 27a and, as previously described, generates signal 37
to establish a switching output from controller 19.
The switching signals thus are applied to the controller 19. The
illustrated controller 19 includes a ring counter 38 to sequentially
recycle the group of six signals to the programmed decoder during each
360.degree. revolution. Thus, the six signals are transmitted as a binary
bit number to the programmed decoder, which decodes the signal and
sequentially energizes the gates 52-54 and for sensing of the respective
windings 4-5-6 and also triggers gates 56 and 61 to alternately transmit
the appropriate positive going signal to the comparator 64 and creations
of signals 37.
Referring particularly to FIGS. 2 and 3, the sequence of operation is
described. FIG. 2 illustrates the operation of the tracking signal circuit
and the timing circuit 25. FIG. 3 illustrates the actuation of the power
switches 13-18, inclusive, to establish sequential pulsed energization of
the windings 4, 5 and 6.
Referring to FIG. 3, the six switches 13 through 18 are identified on the
Y-axis of the graphical illustration, with the closure of the switches
shown by the increased level from reference. Switch 13 closure couples the
positive dc power of supply 7 to winding 4. Switch 14 similarly couples
positive power to winding 4 while switch 15 couples positive power to
winding 6. Switches 16, 17 and 18 are actuated to connect the windings 4,
5 and 6 to the return side of supply 7 to complete the circuit to the
several windings. The switches 13-18, inclusive, are actuated by
controller 19 in accordance with the switching pulse signals 37 from the
timing circuit 25.
Referring to FIG. 2, the timing circuit 25 is driven by the zero crossing
detection circuit 41. The windings 4, 5 and 6 are sequentially
de-energized and have the induced sine wave voltage defining the reference
zero crossings. Each zero crossing creates a pulse signal 66.
Winding 4 is shown creating the first zero crossing, winding 6 the second
and winding 5 the third, with the sequence repeating with continued
forward rotation. The winding being monitored is controlled by the
operation of gated switches 52, 53 and 54 in combination with switches 62
and 63. Thus, switch 52 is closed to detect each zero crossing of the
induced voltage in winding 4, and switches 53 and 54 similarly closed, for
windings 5 and 6. Switches 62 and 63 are alternately closed by the high
and low signals in synchronism with each negative gain signal. Thus, when
the signal to switches 62 and 63 is high, switch 62 transmits the zero
crossing signal and where the signal is low, the inverter 65 closes switch
63 and transmits the zero crossing signal.
Each signal 66 is created at a zero crossing to set reference capacitor 24
and reset timing capacitor 26. The reset timing capacitor 26 is rapidly
reset to zero reference and begins to charge at a constant rate as shown
in FIG. 2. The capacitor charge reaches the reference capacitor charge
level applied to the comparator 27 of FIG. 1, at essentially 30 degrees
after the zero crossing, and creates the trigger or switching signal 37.
As shown in FIG. 3, the train of signals 37 are routed via the controller
to sequentially actuate the power switches 13-18, inclusive. The pulse 37
created by the zero crossing of the winding 4 closes switch 13 to supply
positive power to winding 4, as shown at 70, and opens switch 15 to remove
power from winding 6. Switch 17 is closed to complete the return path, as
shown at 71. Winding 6 is now de-energized and the zero crossing
sequentially is monitored as shown in FIG. 2 and at its zero crossing
creates signals 66 and 37 in the timing circuit and controller to open
switch 17 and close switch 18 to complete the return circuit for the
winding 4 through winding 6, as shown at 72. Thus, winding 5 is now
de-energized and the induced voltage is monitored, as shown in FIG. 2.
The induced voltage in winding 5 passes through a zero crossing sixty
degrees after the previous zero crossing and establishes the switching
signal 37 which is decoded by the decoder to close switch 14 and supply
positive power to winding 5 as shown at 73 and simultaneously turns off
the power 70 from winding 4 by opening of switch 13. Winding 5 is now
energized with the current returned through the still energized switch 18
and winding 6. Sixty degrees subsequent thereto a further signal 37 is
generated by monitoring the induced voltage in the de-energized winding 4.
That signal does not effect switch 14 but rather turns off switch 18 and
turns on switch 16 thereby providing a return path for winding 5 through
winding 4, as shown at 74. The subsequent zero crossing provides for
de-energizing of winding 5 and supplying power to winding 6, as shown at
75. Each six pulse signals 37 provide for the sequential energizing of
windings 4, 5 and 6 for 120 degrees of rotor rotation, at which time the
cycle repeats to maintain the motor rotation.
For reverse rotation, the winding 5 is selected as the initial control
winding such that the timing and power circuit reverses the energizing
sequence to energize windings 5, 4, and 6 in a repetitive sequence to
reverse the rotor rotation.
The capacitor control system of FIG. 1 can be constructed in an integrated
circuit system using a single counter to record a signal proportional to
the period between zero crossing and generating a commutating or switching
signal based on a fixed percentage of such signal. A typical
implementation adapted for IC circuitry is illustrated in FIG. 4.
The motor 77 is shown with the winding 4, 5 and 6 (as in FIG. 1) to
switched power supply including power applying transistor 78 and parallel
stabilizer diodes 78a.
FIG. 4 is a block diagram of the overall circuit. In FIG. 4, the motor
windings 79 are shown connected to a winding selector unit 81 for
sequentially monitoring the induced voltage in the windings. A zero
crossing circuit or detector 82 monitors the particular signal transmitted
via the selector and generates a crossing signal at each zero crossing. A
digital measurement and timing unit 83 records a count equal to time
between each zero crossing and generates a switch signal applied to
controller 84. The integrated circuit thus includes the similar three
basic components of the capacitor system of FIGS. 1-3. The zero crossing
detector 82 includes a comparator 86 connected to the winding selector 81
and the neutral line 85 of stator winding to generate a crossing. The zero
crossing signal is set or latched in a comparator signal latch circuit or
unit 87 of detector 82 to store the signal. In the integrated circuit, the
detector 82 includes a polarity detector 88 and a commutation detector 89
which are coupled to a latch enable unit 90 to control the latching of the
zero crossing signal in the latch unit 87. The polarity detector 88
detects proper polarity in the output of the comparator 86 to insure the
comparator has recovered from the prior condition. The commutater detector
89 detects the initiation of commutation and actuates a timer 91 which
introduces a time delay to allow the current in the winding being
de-energized to rapidly drop to zero. Thus, during the switching cycle,
the current in the de-energized winding discharges through the conducting
return switch 78 and the discharge diode 79 in parallel with the return
switch 78 tied to the winding being de-energized.
The windings 4-6 of motor 77 are highly inductive and energy is stored in
the core, not shown, of the winding during current flow in the winding.
The energy must be dissipated when the winding is turned off to prevent
damage to the system. In accordance with known functioning, the energy is
rapidly dissipated by shorting the winding to ground or other suitable
reference. In the illustrated embodiment, the conducting winding is
connected to reference through the return switch to the common or
reference line and a reverse diode. The current flow thus momentarily
reverses in direction through the winding. The reversal creates a second
zero crossing in the unenergized winding, which would create a false
signal to the controller.
The timer is connected into circuit to effectively cancel the false zero
crossing signal and insure repeated response to the actual zero crossing
of the induced signal.
The comparator latch 87 is enabled when the commutation timer 91 times out
and the polarity detector 88 signals the recovery of the comparator 86.
The zero crossing signal is then latched into the comparator latch unit
87. The output of the comparator latch 87 is also coupled to the latch
enable 90 after the switching is latched to prevent multiple zero crossing
detection during a sampling period as might be caused by ripples in the
waveform being sampled. The latch signal is impressed on a pulse generator
92 having a control line 93 to produce a single, fixed duration timing
pulse, similar to pulse signal 66 of the first embodiment. The timing
pulse is impressed on the digital measurement circuit 83.
The digital measurement circuit 83 includes a counter 94 having a control
connected to a pulse source 95. A reset enable 96 for resetting the
counter 94 is coupled to the pulse generator 92 and to counter 94. The
output of the counter 94 is direct connected to a count latch unit 97
having a set input line 98 connected to the control line 93. The counter
94 is connected to a suitable clock 95 to provide a continuous count. The
counter 94 is reset by unit 96 to initiate a count to reset the counter
and initiate a new count from a reference. The clock may, for example,
operate at 100 KHz providing an angle resolution of 300 Hz. This provides
a count of 27 for each 30 degree intervals. The reset enable unit 96
resets the counter shortly after the reset enable circuit receives a
signal from the zero crossing detector 82. The latch or set input line 98
responds to the zero crossing detector signal in common with the reset
enable circuit latches the count in the counter in response to the
generation of a zero crossing signal just prior to the counter reset. The
latch unit 97 includes a divide-by-two circuit to establish an output
equal to 1/2 the set count. Thus, the total period between zero crossing
is 60 degrees. Therefore, dividing the count number by 2 defines a number
equal to 30 degrees. The reset enable is operative after the momentary
delay period, during which the latching of the count is made, to reset the
counter, and the counter then initiates a new count.
A comparator 100 has a first input connected to the counter output of
counter 94 and to the output of latch unit 97. When the counter number
equals the latch unit number, the comparator 100 generates a commutation
signal at its output. Thus, at the 50 percent count of latch unit 97, the
rotor has rotated 30 degrees from the last zero crossing, and the induced
signal intercept of the winding voltages is present, at which positive
power is to be applied to the unenergized windings and simultaneously
power is removed from the preceding phase winding.
At the next zero crossing, the above cycle is repeated with the resetting
of the latch register and the resetting of the counter to again initiate a
new count and measurement of the next zero crossing period.
The commutation pulse signal is applied to the programmer 84 which includes
a divide-by-six counter unit 101. The count is divided by six to recycle
each complete revolution during which the windings have been pulsed for
the respective 120 degree periods of positive power application and
appropriately switched to form the commutating return paths.
The decoder has a direction forward/reverse (F/R) input unit 102 coupled to
a suitable manual or automated signal source, as in the first embodiment,
to set the sequence selection of the windings for forward or reverse
rotation.
The programmer is connected to a start-up pulse unit for initiating the
rotation of the rotor with the measurement circuit disabled.
A start-up unit is coupled to the decoder unit to generate a series of
apparent zero crossing signals which are time spaced to initiate the motor
rotation. The start-up circuit produces the necessary signals to
accelerate the rotor and thereby induce suitable voltages in the
unenergized winding for generating the zero crossing signals. The pulse
signals are preferably progressively increased in frequency in accordance
with anticipated increase of acceleration of the motor. After a short
period, such as one second, the rotor should be at a proper speed to
induce a voltage in the unenergized winding at a suitable level for
automatic generation of operative zero crossing signals. The generating of
the start-up pulses ceases, the counter is coupled to sense the actual
zero crossing, the windings are sequentially monitored and power is
supplied in accordance with the zero crossings.
In the initial start mode, the motor is thus operated in a stepping mode
with sufficient current supplied in time spaced pulses for driving of the
motor at torque angles significantly less than 90.degree.. In many
applications such as ventilation loads, pump loads and the like, the
torque is proportional to speed squared, thereby requiring relatively low
torque input for the starting low speeds. A proper current can therefore
be supplied to the motor without danger to the motor.
The output of the programmer includes the six gate signal lines 103 for
selectively enabling the gated power switches 78, and winding selector
lines 104 for selectively enabling the selector 81 for sensing the output
of the unergized winding. The system of FIG. 4 thus sequentially
de-energizes one winding and energizes the next proper winding at the
precise time to create an optimum motor torque.
Various modes of carrying out the invention are contemplated as being
within the scope of the following claims particularly pointing out and
distinctly claiming the subject matter which is regarded as the invention.
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